Forest Garden Greenhouse Construction

Our forest garden greenhouse is based on the work of Jerome Osentowski from Colorado USA. Over many years he has pioneered the use of ground heat storage systems (that he calls ‘climate batteries’) to store heat from summer and autumn to use in winter, to enable a growing system for subtropical or tropical plants.

Our greenhouse was erected in summer 2016, after initial groundworks involving burying air pipes in the ground (air is blown by fans through these to store or release heat). In addition to the soil heat storage we have also designed a ‘water wall’ where a large volume of water can also function as a heat store. Water is quicker and more efficient at storing heat than soil, but it also releases heat quicker. The idea is that the water wall evens out day-to-day variations in temperature, whilst the soil storage evens out longer-term variations.

As far as we are aware nobody has set up this kind of system in the UK with a monitoring system to record not just above ground climatic variables but also below ground. Some of that data will appear on our web pages in real time, as well as historical records.

The ideal design for these systems uses a southern slope for the site and digs away to make level ground with an earth wall at the northern (back) side of the greenhouse. However, our first challenge was that the site (at our Littlehempston research site) is on a slight north slope.

So an earth wall on the north side of the building was not feasible. Instead, we decided to create our own wall there instead. After investigating the feasibility, cost, and heat storage characteristics of a stand-alone solid wall (eg. Built out of standard aggregate blocks, or built out of cob) this was discounted mainly from a cost perspective. An alternative which we’d mulled for some years was a “water wall” made up of stackable cubic ibc water tanks. These tanks are approx. 1m cubed and on metal pallets can be stacked up to 4 high full of water.

The obvious choice was to use black ibc tanks so that the water inside did not get full of algae etc. – especially important as some of the tanks could be used for irrigation water.

We planned the wall to use up to 3 tanks height and these, when full of water, would weight 1 ton each – hence underneath would need to be a concrete base that could cope with that load. One disadvantage of requiring black ibc’s is that there are few available second hand at low cost, compared with white/translucent tanks. We ended up buying reconditioned tanks that had already been used but had then been cleaned and were in good condition.

As for the design of the greenhouse itself, we decided not to design our own, but to use a standard commercial “Venlo” type glasshouse. This would be much cheaper than a bespoke building, and there were a couple of companies in the UK who specialised in erecting second hand Venlo glasshouses which were much cheaper than buying new.

The size of the glasshouse depended on the space available on site and the cost: we decided to go for a building 22.5m x 19.2m in size, with height of 6.75m to eaves (6.0m to the top of the walls.) This is pretty high for a Venlo structure but the extra height will allow us to grow quite large trees inside.

First works started in May 2016. The groundwork excavations were undertaken by Graham Soper, a local contractor with over 50 years of experience (compared to our clumsy efforts when hiring a mini-digger, he was like an artist with his machines.)
Looking East, the north facing slope can clearly be seen. Mainly a ‘cut and fill’ job, with the spoil cut out from the slope used to build up the lower section. At the top, about 1.5m depth has been cut away, and the lower section built up by a similar amount. Graham used a laser level to level the area to within a few cm.

Underground air pipes

The next stage of the groundworks involved creating and burying the network of ground air pipes that would carry fan-blown air through under soil level, to store or release heat in the greenhouse.

We decided that we would use two layers of pipes, at depths of 1.0m and 0.5m. Going deeper has some advantages (more thermal stability, and a greater thermal capacity) but also disadvantages (more difficult to excavate holes). In a colder winter climate than Devon there may be more rationale for going deeper too, but here, where average temperatures in January are about 7°C, we felt that 1m would be deep enough.

Layout of the ground pipes required quite a lot of research and design ideas. Diameter of the pipes and their length depends on the fans being used to blow air down them, as there is an optimal air velocity for exchanging heat between the pipes and soil. In addition, larger diameter pipes become very expensive so there are cost implications in the equation too.

We decided to lay the ground pipes in 3 separate trenches, with each having its own fan. Each trench was about 18m x 6m, dug 1m deep.

Underground air pipes

Ground pipe installation

Each ground pipe installation consisted of a larger diameter (30cm) feeder pipe, with smaller laterals (15cm diameter soil drainage pipe) making a finer network. Two layers of laterals at 1m and 0.5m deep both connected to the feeder pipe. The soil drainage pipe used for the laterals was the type with perforations – this is important, as warm humid air blown through the pipes will make condensation form inside them, andthis must be allowed to drain away freely.

Unlike Jerome Osentowski’s designs, the laterals do not then join into a larger diameter return pipe. Instead, they rise (in clusters) into the greenhouse space at regular intervals.

The 1m deep layer of laterals was installed first, then 0.5m of soil backfilled (this is what is happening in the photo above.) Then the second layer of laterals was installed, before the final 0.5m of soil was backfilled. Because the area of the three pipe trenches was significantly less than the overall site area, the whole 1m of backfill was topsoil, leading to twice the depth of topsoil than was originally on the site.

This photo is looking West. The painted line shows the external edges of the glasshouse. The central trench is being backfilled, whilst the trench nearest the camera is partly backfilled.

Steel frame being constructed

The company constructing the glasshouse had some minor groundworks of their own to do in the summer, digging holes for the main posts and a trench around the boundary for a low block wall.

The steel posts, 6cm x 6cm by 6m high, were concreted into the ground, and a one-block high boundary wall built.

You can also see in the photo three rectangular constructions. No, they’re not graves, but are housings for the three fans used with the ground pipes. The fans are designed to sit underground to minimise noise.

Steel frame being constructed

Steel frame being constructed

It turned out that the glasshouse company had never actually built a glasshouse this high before (!) but the standard design meant that they had few problems. One factor we had not foreseen was how much they would need to drive the two 5-ton scissor lifts over the topsoil, resulting in significant soil compaction and (possibly) some damage to some of 0.5m deep ground pipes.
Here they are using scissor lifts to erect the frame.

Polycarbonate cladding

The Venlo design uses standard width bays (6.4m each) with two apexes per bay (hence our building width of 3 x 6.4m = 19.2m). Our building was to be glazed mainly with twin-wall polycarbonate (6mm thick), with toughened glass in the opening roof windows. Thicker polycarbonate would have been nice from an insulation perspective, but the Venlo design cannot accommodate it. But even 6mm thick polycarbonate is a better insulator than glass, although the light penetration qualities are not quite as good.

Four of the six roof ridges are visible in this photo. Glazing bars hold the roof polycarbonate in place and some of the glass windows (square as opposed to long rectangles of polycarbonate) are visible. One of the exterior doors has been fitted.

We were fortunate through August and September that the weather was quite fine and settled, so there were few delays for rain or wind. One concern we certainly had was how the structure would cope with the stormy winter winds which we frequently experience in Devon. The site is sheltered from the South East and South, and somewhat sheltered from the South West and West: these are where our strongest winds come from. There is exposure to the North and East. We were soon to get some testing winds from these directions.

Polycarbonate cladding

Glazing almost complete

Glazing almost complete

After glazing most of the roof, the sides were then glazed with polycarbonate. The glazing is attached to the horizontal metalwork with glazing clips.

Then, two days from the glazing being finished, disaster! There was a theft of the entire glasshouse company’s tools from the site, the thieves breaking in through a neighbour’s field. The workers headed back to Yorkshire, unable to finish the job until they had bought a whole new set of tools. [there was no prospect of getting the tools back. The Police reckoned that by the next day the tools would be 1000 miles away.]

In the meantime the weather became more autumnal. A storm came in and did some damage to the roof glazing which had not been completed. Luckily the damage was slight, and when the workers returned from Yorkshire 2 weeks later everything was put right and the structure quickly finished.

Water wall

Now we could start to install the water wall. During a lull in the glasshouse construction, we had laid the concrete base for it, 1.2m wide by the whole 22.5m length of the structure, against the north wall.

The first layer of ibc tanks was filled with water (using reservoir water from our own reservoirs – didn’t need to be particularly clean as the water would just stay there) before the second layer was added.

This photo shows the first layer of ibc tanks in the water wall, and ground pipe risers.

Second layer of water tanks in place.

You can also see in this photo that insulation sheets were placed vertically behind the water wall, to stop heat stored in the wall from disappearing out through the glasshouse polycarbonate wall.

You can also see a riser emerging from one of the fan housing for the ground pipes. This draws in air from mid-height (3m high) in the glasshouse before blowing it down through the ground pipes. You can see clusters of smaller risers emerging along the length of the structure, where the warmed or cooled air vents back into the glasshouse.

Water wall

Second layer of water tanks

If you look carefully in the upper part of this photo you can also see two fans mounted high in the glasshouse. These are destratification fans, specially designed to mix up the air inside, so it doesn’t get really hot high up. They draw air in from both below and above, and thrown it sideways out from the fan. The idea is that they are left running the whole time, and as well as mixing air to even out the temperature, they keep the air moving which is beneficial to plants. The moving air also helps liberate or store heat in the water wall. These fans run on electricity of course, and the aim in time is to install a solar electric system to power these and the ground pipe fans.

The ladder in the photo is giving access to the motor which opens and closes the roof windows.

Final layer of water tanks

The second layer of tanks was filled with water and the third layer placed on top. The empty tanks weight about 60kg so getting these up there took a bit of muscle power (thanks Jeremy and Laurie). You might wonder why the wall is not continuous at 3 tanks high.

Well, there needs to be access where the ladder is up to the roof motor, so only 1 tank fitted there. The gap further along is because in time there will be two tanks used for aquaculture.

Final layer of water tanks

Soil decompaction

Soil decompaction
We also needed to address the soil compaction caused by the scissor lifts during construction. Whilst plants can in time undo much of this kind of damage, it can still greatly aid the growing system for the first few years if some physical decompaction can take place.
We decided to get a company that specialises in soil decompaction to come and use a machine on site. The photo below shows them using the Terravent machine, which sinks a probe into the soil, then injects a burst of high pressure air. This breaks up the decompacted layer (you can see the ground moving upwards and cracks appearing as the air is injected). These machines are used at places like Kew Gardens to decompact around trees, when as well as injecting air, mycorrhizal fungi are injected as well.